Phenolic resins can be produced from partially reacted resols. Resols are low polymers or oligomers that may be produced by the reaction of phenols with aldehydes, using an excess of aldehyde. The starting materials for use in the manufacture of resols which are used in greatest volume are phenol and formaldehyde. However, other commonly used starting materials include alkyl-substituted phenols such as cresols, xylenols, p-tert-butyl-phenol, p-phenyl-phenol, nonylphenol, and the like. Also diphenols, for example, resorcinol and bisphenol-A. In addition to formaldehyde, other aldehydes can be used, such as acetaldehyde or furfuraldehyde can also be used.
Typically, phenol and formaldehyde are reacted in the presence of a basic catalyst, such as sodium hydroxide or potassium hydroxide, followed by neutralizing the solution and distilling off water. The initially produced resol is commonly referred to as an A-stage resin. This A-stage resin can be reacted further in the presence of an acid catalyst to form a phenolic resin, which is often referred to as curing. During curing, some formaldehyde and water are usually liberated. In particular, the resols (A-stage resins) contain reactive methylol groups that can react further to enlarge the polymeric chain length and/or cross-link to form a three-dimensional network. If the curing is carried out in the presence of a blowing agent, a phenolic resin foam can be produced.
Phenolic resin foams are well known and provide many advantages over polyurethane foams. For example, polyurethane foams produce many toxic fumes when burned, whereas phenolic foams produce significantly less toxic fumes when burned. However, polyurethane foams can be formed having a closed-cell structure. A closed-cell structure is understood as the ability of the cell wall to inhibit the outward diffusion of trapped blowing gas and the inward diffusion of air. Such closed-cell polyurethane foams can be produced under atmospheric pressure.
In contrast, conventional phenolic resin foams must be produced under high pressure to form such a closed-cell structure. U.S. Pat. No. 4,423,163 describes such a high pressure method for making a closed-cell phenolic resin foam, in which the foam is produced under about 6 psi. The requirement of pressure for making a closed-cell phenolic foam results in significant disadvantages and is not commercially feasible, especially when compared to closed-cell polyurethane foams that can be produced under atmospheric pressure.
Thus, there is a great need for a method for making a closed-cell phenolic resin foam that does not require the use of pressure greater than about atmospheric. There is also a need for a foamable composition that is adapted to provide a closed-cell phenolic resin foam, even when blown and cured under atmospheric pressure.